The present invention pertains to a radiation-curable white inkjet ink and, more particularly, to a radiation-curable white inkjet ink comprising a polymerizable vehicle and a titanium dioxide which is dispersed by at least two dispersants that is curable on exposure to radiation such as ultraviolet light.
Inkjet imaging techniques have become very popular in commercial and consumer applications. Ink jet printers operate by ejecting ink onto a receiving substrate in controlled patterns of closely spaced ink droplets. By selectively regulating the pattern of ink droplets, ink jet printers can produce a wide variety of printed features, including text, graphics, images, holograms and the like. Moreover, ink jet printers are capable of forming printed features on a wide variety of substrates, as well as three-dimensional objects in applications such as rapid prototyping.
Inkjet inks must meet stringent performance requirements in order for the inks to be appropriately jettable and for the resultant printed features to have the desired mechanical, chemical, visual and durability characteristics. In particular, inks must have relatively low viscosity when jetted, yet must be able to form accurate, durable images on the desired receiving substrate. For example, a typical ink will have a viscosity in the range of 3 to 30 centipoise at the jetting temperature. The low viscosity, however, poses a substantial challenge to achieving printed features with good mechanical and durability characteristics.
Aqueous based inks, although very successful in the home and small office market, have certain drawbacks in other applications. For example, in industrial printing, the substrate is typically nonporous and the aqueous ink must be dried, which is equipment intensive and time consuming. Also, the printed material needs to be handled carefully during the relatively lengthy drying period. Water-based inks are also compatible only with a limited range of substrates, and images formed using water-based inks typically require a protective overlaminate for outdoor applications.
Solvent-based inks, which are most commonly used in industrial applications, contain relatively volatile organic solvents. These inks dry more readily than aqueous inks and tend to be somewhat more durable in outdoor applications. However, the solvents require careful handling and may be toxic and/or flammable. These inks also tend to be compatible with only a limited range of substrates.
To avoid using conventional solvents, inks with polymerizable diluent have been developed. The diluent generally comprises one or more reactive monomers that are polymerized by exposure to radiation (radiation curable) such as ultraviolet light, electron beam energy and the like. The cured diluent forms a polymer film that provides durability to the print and requires no drying. The diluent mixture, which is also the ink vehicle, is chosen to provide appropriate ink viscosity. The reactive monomer content in the diluent also impacts the physical properties of the printed image (durability, flexibility, elasticity, gloss, hardness, chemical resistance, stiffness and so forth) and is optimized to achieve the desired properties.
Inkjet inks must possess proper dot gain in order to form images with attractive appearance. Dot gain refers to the degree to which an ink jetted drop spreads out upon application to a substrate. If an ink jetted drop spreads out too much on the substrate, then poor edge definition and intercolor bleed is observed. On the other hand, if an ink jetted drop spreads insufficiently upon application to the substrate, poor color density results. Dot gain characteristics depend upon a number of factors including the ink jet composition, the nature of the substrate, the substrate temperature, and the interfacial tension between the ink and the substrate. Many of the currently available radiation curable inks show favorable dot gain characteristics on some substrates, but not on others. Preferably a curable inkjet ink would be formulated to provide good dot gain characteristics and good appearance on a broad range of substrates.
There are numerous descriptions of white inks formulations that are claimed to be adequately formulated for use in ink jet system. U.S. Pat. No. 5,439,514 describes an ink containing both a pigment and titanium dioxide or alumina. U.S. Pat. No. 6,769,766 a white ink and colored UV inks where the tank for white ink is larger than the other tanks and agitated. U.S. Pat. No. 6,989,054 describes an aqueous slurry of a phosphate treated titanium dioxide and a specific particle size distribution. U.S. Pat. No. 7,026,368 a titanium dioxide which is treated with silica and the polymeric dispersant used has a basic functional group. U.S. Pat. No. 6,593,390 has an example using a titanium dioxide white pigment and a single basic dispersant group. Co-owned U.S. patent application Ser. Nos. 10/872,856 (filed Jun. 21, 2004), Ser. No. 11/070,714 (filed Mar. 2, 2005), and 60/717,438 (filed Sep. 15, 2005) all describe white inks suitable for certain ink jet needs (the disclosures of which are incorporated by reference herein for all purposes as if fully set forth).
It is, therefore, desirable to provide radiation curable white inks with improved characteristics, such as image durability and/or cure speed, and that are also suitable for use on a broad range of substrate types.